Heretofore, in the field of radio communication system, HSDPA has been standardized whereby a plurality of communication terminals share a high-speed, high-capacity downlink channel and perform high-speed packet transmission in the downlink (i.e. High Speed Downlink Packet Access) (see, for example, Laid-Open Japanese Patent Application Publication No.2000-151623)
In the HSDPA system, the communication terminal transmits the signal called CQI (Channel Quality Indicator) representing the modulation method and spreading factor for packet data that allow demodulation in the communication terminal, in a cycle set by higher apparatus such as RNC (Radio Network Controller). The base station receives the CQI, and, based on the CQIs from individual communication terminals, performs scheduling and selects the optimal modulation method, spreading factor, and others. Then, the base station modulates and encodes transmit data in accordance with the modulation method and spreading factor that are selected, and, based on the scheduling result, transmits data to the individual communication terminals. By this means, it is possible to adaptively change the transmission rate depending on the signal propagation environment and transmit great amount of data from the base station to the communication terminal.
As to the method of transmitting the CQI, there is a method standardized in 3GPP, TS 25.214 V5.5.0 6A.1.2. According to this method, the communication terminal calculates the CQI on a regular basis based on the parameter called “feedback cycle k” and transmits the calculated CQI to the base station. In addition, the communication terminal repeats transmitting the CQI to the base station for the number of times determined based on the parameter called “N_CQI_transmit”.
FIG. 1 shows a configuration of a conventional communication terminal that transmits the CQI. In communication terminal 10, radio receiver 13 receives a radio signal that is transmitted from a radio base station, via antenna 11 and antenna duplexer 12, performs predetermined radio processing, and thereafter transmits the result to despreader 14. Despreader 14 performs despreading processing on the signal inputted from radio receiver 13, and sends the signal after the despreading processing to demodulator 15 and SIR measurer 17. Demodulator 15 performs demodulation processing on the signal inputted from despreader 14 and sends the signal after the demodulation to decoder 16. Decoder 16 performs decoding processing on the signal after the demodulation and obtains the received data. In addition, SIR measurer 17 measures the SIR (Signal to Interference Ratio) of the signal inputted from despreader 14, and sends the measured SIR to CQI calculator 18. Based on the measured SIR, CQI calculator 18 determines downlink transmission rate information (i.e. CQI) that allows reception in communication terminal 10 and sends the result to transmit frame generator 19.
Based on CQI update cycle information and CQI repetition count information stored in memory 24, CQI transmission timing controller 20 selects the CQI to transmit and the transmission timing of the CQI. In practice, CQI transmission timing controller 20 changes content of the CQI in a cycle in accordance with the parameter feedback cycle k (i.e. CQI update cycle information), which is stored in memory 24, and sends a control signal, which commands to transmit the same CQI for the number of times in accordance with the parameter repetition (i.e. CQI repetition count information), which is stored in memory 24, to transmit frame generator 19. Incidentally, the CQI update cycle information and CQI repetition count information stored in memory 24 are configured by higher apparatus such as RNC, and are received via the radio base station.
FIG. 2 shows CQI transmission timings in the communication terminal where feedback cycle k is 3 (meaning that the CQI is calculated once every three sub-frames, and that the CQI is changed every three sub-frames and transmitted to the radio base station) and repetition is 2 (meaning that the same CQI is transmitted twice in consecutive sub-frames).
In the periods in which the CQI is calculated (i.e. measurement reference periods), the communication terminal measures CQI 1 (FIG. 2(a)) in measurement period Ref 1, which corresponds to sub-frame (SF) #0, and repeats transmitting CQI 1 in SF #1 and SF #2 in HS-DPCCH (High Speed-Dedicated Physical Control Channel) sub-frames' (FIG. 2(b)). Likewise, the communication terminal measures CQI2 in measurement period Ref 2, which corresponds to SF #3, and repeats transmitting CQI 2 in SF #4 and SF #5. Incidentally, SF #0-SF #5 are formed with three slots each, designed such that the ACK/NACK signal is embedded in the first one slot and the CQI signal is embedded in the other two slots.
Transmit frame generator 19 generates a transmit frame from transmit data and the CQI signal and sends the result to modulator 21. In practice, as mentioned above, in accordance with the control signal from CQI transmission timing controller 20, transmit frame generator 19 determines the position in the transmit frame where the CQI signal is embedded, and determines whether to embed the same CQI signal or embed a new, changed CQI signal, and generates the transmit frame.
Modulator 21 modulates the transmit frame inputted from transmit frame generator 19 and sends the result to spreader 22. Spreader 22 spreads the signal after the modulation and sends the result to radio transmitter 23. Radio transmitter 23 performs predetermined radio processing on the signal after the spreading and transmits the result to the radio base station via antenna duplexer 12 and antenna 11.
FIG. 3 shows a configuration of a conventional radio base station that receives the CQI signal from communication terminal 10 and transmits downlink signals based on the received CQI. In radio base station 30, radio receiver 33 receives a radio signal that is transmitted from communication terminal 10 via antenna 31 and antenna duplexer 32, performs predetermined radio processing, and thereafter sends the result to despreader 34.
Memory 44 stores the same CQI update cycle information and CQI repetition count information as those stored in memory 24 in communication terminal 10. Consequently, CQI reception timing controller 35 determines the timing to receive the CQI from the same CQI update cycle information and CQI repetition count information as those used in communication terminal 10, and sends reception timing information to despreader 34. CQI reception timing controller 35 determines the number of times the CQI is combined from the same CQI update cycle information and CQI repetition count information as those used in communication terminal 10, and sends combining count information to buffer 37 and decoder 38.
FIG. 4 shows CQI reception timings in radio base station 30, with the same parameters as in communication terminal 10—that is, CQI feedback cycle k is 3 and repetition is 2. In this case, sub-frame (SF) #1 and SF #2 have a timing to receive CQI 1 and SF #4 and SF #5 have a timing to receive CQI 2. CQI 1 and CQI 2 are each combined twice.
Despreader 34 despreads the signal inputted from radio receiver 33 in accordance with the CQI reception timing indicated by CQI reception timing controller 35. Demodulator 36 demodulates the signal inputted from despreader 34 and sends the demodulation result to buffer 37. Of the signals inputted from demodulator 36, buffer 37 keeps the CQI signal and sends the rest of the signals to decoder 38.
Buffer 37 holds the CQIs in an equivalent number as the CQI combining count indicated by CQI reception timing controller 35, sends the CQI signals held to decoder 38, and thereafter erases the content of buffer 37. In FIG. 4, when the number of the CQI signals held reaches two, buffer 37 outputs these CQI signals to decoder 38 and erases the content of buffer 37.
Decoder 38 decodes the signal after the demodulation inputted from buffer 37, and obtains the received data. In addition, decoder 38 combines and decodes the CQI signals inputted from buffer 37 in accordance with the CQI combining count indicated by CQI reception timing controller 35, and sends the decoded CQI to scheduler 39. The CQI signals are decoded by the CQI combining count, meaning that all the CQI signals outputted from buffer 37 equal to the combining count in number, are combined and decoded. In FIG. 4, the two CQI signals held in buffer 37 are combined decoded to obtain one decoding result.
Scheduler 39 determines the transmission rate of the transmit data based on the CQI inputted from decoder 38 and sends the result to transmit frame generator 40. Transmit frame generator 40 generates a transmit frame based on the transmission rate reported from scheduler 39, and sends the result to modulator 41. Modulator 41 performs modulation processing on the signal inputted from transmit frame generator 40 and sends the modulated signal to spreader 42. Incidentally, the modulation method in modulator 41 can be changed depending on the transmission rates. Spreader 42 performs spread modulation on the signal after the modulation and sends the spreading result to radio transmitter 43. Radio transmitter 43 performs predetermined radio processing on the signal after the spread-modulation and transmits the result to communication terminal 10 via antenna duplexer 32 and antenna 31.
However, as mentioned above, in a system where the CQI update cycle and CQI repetition count are designated, problems might occur depending on the combinations of the CQI update cycle and CQI repetition count. FIG. 5 illustrates an example of such problem. FIG. 5 shows CQI transmission timings in the communication terminal, where CQI feedback cycle k is 2 (meaning that the CQI is calculated once every two sub-frames and the CQI is changed every two sub-frames and sent to the radio base station), and repetition is 3 (meaning that the same CQI is transmitted three times in consecutive sub-frames).
In the periods in which the CQI is calculated, the communication terminal measures CQI 1 in measurement period Ref 1, which corresponds to sub-frame (SF) #0, and repeats transmitting CQI 1 in SF #1, SF #2, and SF #3 in HS-DPCCH sub-frames. In addition, CQI 2 is measured in measurement period Ref 2, which corresponds to SF #2, and transmitted in SF #3, SF #4, and SF #5.
As a result, a transmission timing of CQI 1 and a transmission timing of CQI 2 overlap in SF#3. Moreover, it is not clear whether CQI 2 is transmitted in SF #4. Likewise, a transmission timing of CQI 2 and a transmission timing of CQI 3 overlap in SF #5, and a transmission timing of CQI 3 and a transmission timing of CQI 4 overlap in SF #7.
This creates the problem that the communication terminal becomes unable to determine which CQI to transmit in the sub-frames where transmission timings overlap. If the communication terminal blindly selects one of the CQIs and transmits it, this might result in a case where the received power of the CQI obtained in the base station apparatus by means of combining, is insufficient and the CQI is restored erroneously. If the error rate of the CQI increases, error rate characteristics in the downlink will also deteriorate, and, consequently, the amount of traffic in the downlink decreases.